Digital watermarking technology, a form of steganography, encompasses a great variety of techniques by which plural bits of digital data are hidden in some other object preferably without leaving human-apparent evidence of alteration.
Digital watermarking modifies media content to embed a machine-readable code into the data content. The data may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media such as images, audio signals, and video signals. However, it may also be applied to other types of data, including text documents (e.g., through printing, line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects. Of course, images, graphics, background patterns, seals, logos and artwork can be watermarked and then printed or applied to documents or other objects.
Other techniques extend the texturing techniques, e.g., by employing an intaglio press to texture the media as part of the printing process (either without ink, or with clear ink).
Printable media—especially for security documents (e.g., banknotes, checks, etc.) and identification documents (e.g., passports, driver's licenses, bank cards, visas, government issued documentation, company cards or badges, photograph identification cards, etc.)—are ideally suited to receive digital watermarking. The terms “ID document” and “card” are sometime interchangeably used for “identification document.”
Digital watermarking systems have two primary components: an embedding component that embeds a watermark in media content, and a reading component that detects and reads the embedded watermark. The embedding component embeds a watermark pattern by subtly altering data samples of the media content. The alterations usually take the form of altered signal values, such as slightly changed pixel values, picture luminance, picture colors, changed DCT coefficients, instantaneous audio amplitudes, etc. However, a watermark can also be manifested in other ways, such as changes in the surface microtopology of a medium, localized chemical changes (e.g. in photographic emulsions), localized variations in optical density, localized changes in luminescence, etc. The surface texture of an object may be altered to create a watermark pattern. This may be accomplished by manufacturing an object in a manner that creates a textured surface or by applying material to the surface (e.g., an invisible film or ink) in a subsequent process. The watermark reading component analyzes content to detect whether a watermark pattern is present. In applications where the watermark encodes information, the reading component extracts this information from the detected watermark.
The watermark components may be located in different spatial or temporal locations in a host signal. In images, for example, different components may be located in different parts of the image. Each component may carry a different message or perform a different function.
The watermark components may be defined, embedded and extracted in different domains. Examples of domains include spatial and frequency domains. A watermark may be defined in a domain by specifying how it alters the host signal in that domain to effect the encoding of the watermark component. A frequency domain component alters the signal in the frequency domain, while a spatial domain component alters the signal in the spatial domain. Of course, such alterations may have an impact that extends across many transform domains.
In addition, components may be located in different spatial portions of the host signal, and may carry the same or different messages.
The host signal can vary as well. The host is typically some form of multi-dimensional media signal, such as an image? audio sequence or video sequence. In the digital domain, each of these media types is represented as a multi-dimensional array of discrete samples. For example, a color image has spatial dimensions (e.g., its horizontal and vertical components), and color space dimensions (e.g., YUV or RGB). Some signals, like video, have spatial and temporal dimensions. Depending on the needs of a particular application, the embedder may insert a watermark signal that exists in one or more of these dimensions.
While described here as watermark components, one can also construe the components to be different watermarks. This enables the watermark technology described throughout this document to be used in applications using two or more watermarks. For example, some copy protection applications of the watermark structure may use two or more watermarks, each performing similar or different functions. One mark may be more fragile than another, and thus, disappear when the combined signal is corrupted or transformed in some fashion. The presence or lack of a watermark or watermark component conveys information to the detector to initiate or prohibit some action, such as playback, copying or recording of the marked signal. Or portions of the various watermark components can be compared for authentication.
A watermark system may include an embedder, detector, and reader. The watermark embedder encodes a watermark signal in a host signal to create a combined signal. The detector looks for the watermark signal in a potentially corrupted version of the combined signal, and computes its orientation. Finally, a reader extracts a message in the watermark signal from the combined signal using the orientation to approximate the original state of the combined signal.
In the design of the watermark and its components, developers are faced with several design issues such as: the extent to which the mark is impervious to jamming and manipulation (either intentional or unintentional); the extent of imperceptibility; the quantity of information content; the extent to which the mark facilitates detection and recovery, and the extent to which the information content can be recovered accurately.
For certain applications, such as copy protection or authentication, the watermark is preferably difficult to tamper with or remove by those seeking to circumvent it. To be robust, a watermark preferably withstands routine manipulation, such as data compression, copying, linear transformation, flipping, inversion, etc., and intentional manipulation intended to remove the mark or make it undetectable. Some applications require the watermark signal to remain robust through digital to analog conversion (e.g., printing an image or playing music), and analog to digital conversion (e.g., scanning the image or digitally sampling the music). In some cases, it is beneficial for the watermarking technique to withstand repeated watermarking.
A variety of signal processing techniques may be applied to address some or all of these design considerations. One such technique is referred to as spreading. Sometimes categorized as a spread spectrum technique, spreading is a way to distribute a message into a number of components (chips), which together make up the entire message. Spreading makes the mark more impervious to jamming and manipulation, and makes it less perceptible.
Another category of signal processing technique is error correction and detection coding. Error correction coding is useful to reconstruct the message accurately from the watermark signal. Error detection coding enables the decoder to determine when the extracted message has an error.
Another signal processing technique that is useful in watermark coding is called scattering. Scattering is a method of distributing the message or its components among an array of locations in a particular transform domain, such as a spatial domain or a spatial frequency domain. Like spreading, scattering makes the watermark less perceptible and more impervious to manipulation.
Yet another signal processing technique is gain control. Gain control is used to adjust the intensity of the watermark signal. The intensity of the signal impacts a number of aspects of watermark coding, including its perceptibility to the ordinary observer, and the ability to detect the mark and accurately recover the message from it.
Gain control can impact the various functions and components of the watermark differently. Thus, in some cases, it is useful to control the gain while taking into account its impact on the message and orientation functions of the watermark or its components. For example, in a watermark system described below, the embedder calculates a different gain for orientation and message components of an image watermark.
Another useful tool in watermark embedding and reading is perceptual analysis. Perceptual analysis refers generally to techniques for evaluating signal properties based on the extent to which those properties are (or are likely to be) perceptible to humans (e.g., listeners or viewers of the media content). A watermark embedder can take advantage of a Human Visual System (HVS) model to determine where to place a watermark and how to control the intensity of the watermark so that chances of accurately recovering the watermark are enhanced, resistance to tampering is increased, and perceptibility of the watermark is reduced. Such perceptual analysis can play an integral role in gain control because it helps indicate how the gain can be adjusted relative to the impact on the perceptibility of the mark. Perceptual analysis can also play an integral role in locating the watermark in a host signal. For example, one might design the embedder to hide a watermark in portions of a host signal that are more likely to mask the mark from human perception.
Various forms of statistical analyses may be performed on a signal to identify places to locate the watermark, and to identify places where to extract the watermark. For example, a statistical analysis can identify portions of a host image that have noise-like properties that are likely to make recovery of the watermark signal difficult. Similarly, statistical analyses may be used to characterize the host signal to determine where to locate the watermark.
Each of the techniques may be used alone, in various combinations, and in combination with other signal processing techniques.
In addition to selecting the appropriate signal processing techniques, the developer is faced with other design considerations. One consideration is the nature and format of the media content. In the case of digital images, for example, the image data is typically represented as an array of image samples. Color images are represented as an array of color vectors in a color space, such as RGB or YUV. The watermark may be embedded in one or more of the color components of an image. In some implementations, the embedder may transform the input image into a target color space, and then proceed with the embedding process in that color space.
In most embodiments, a watermark payload is uniform across the medium. In some applications, however, it may be desirable to encode different payloads in different regions of a medium, or to convey different payloads through different digital watermarks or watermark components.
In other arrangements, the same watermark may be encoded in different places (e.g., on front and reverse sides of a document). A different embedded pattern can be used in different places to encode the same watermark payload.
The watermark can convey a payload of arbitrary length, commonly in the 2-256 bit range, and perhaps most commonly between 24 and 72 bits. Error correcting coding, such as convolutional coding or BCH coding, can be employed to transform a base payload (e.g., 52 bits) to a longer data string (e.g., 96-1024 bits), assuring robustness in detection notwithstanding some data corruption (e.g., due to wear and tear of the medium, artifacts from scanning, etc.). The bits of this longer string are mapped, e.g., pseudo-randomly, to define the pattern (e.g., checkerboard or tiled).
Several particular digital watermarking techniques have been developed. The reader is presumed to be familiar with the literature in this field. Some techniques for embedding and detecting imperceptible watermarks in media signals are detailed in the assignee's U.S. Pat. Nos. 6,122,403 and 6,614,914 and International Application No. PCT/US02/20832 (published as WO 03/005291), which are each herein incorporated by reference.
One aspect of the present invention is an age verification system and process. A document includes auxiliary information steganographically embedded therein. The auxiliary data is used to verify a document bearer's age and whether the bearer is an authorized bearer of the document. The auxiliary data may include or link to biometric information that is associated with an authorized bearer of the document. The data may further include or link to information corresponding to the document bearer's age.
Another aspect of the present invention is a shelf-life identification document. The self-life identification document provides an expiration indicator that becomes evident with use or time. In one implementation we provide a digital watermark through low adhesive inks. The ink degrades or rubs off with use. The watermark is lost as the ink degrades. In a related implementation, a digital watermark is provided on a document surface with relatively high-adhesive ink. The digital watermark is overprinted with a second, relatively low-adhesive ink. The second ink degrades or rubs off with time or use, thus revealing the digital watermark below. The absence or presence of a digital watermark on a shelf-life identification document provides an expiration trigger.
One aspect of the present invention relates to identifying different regions in image data. For example, the image data may correspond to a human subject (e.g., as expected with a passport or driver's license photograph). A face locator is used to identify an image region corresponding to a face or human silhouette. Once identified, the image data can be realigned to center the face or silhouette within a predetermined area or in a center of an image frame. A digital watermark can be embedded in the realigned image data. Or a first digital watermark component can be embedded in the face region or silhouette, while a second digital watermark component can be embedded in a background portion (e.g., an image portion which does not include the face region or silhouette). The first and second digital watermark components can be correlated for authentication.
Another aspect of the present invention relates to authenticating and identifying digital images. A digital image is captured, e.g., at a DMV location, depicting a human subject. A so-called reversible digital watermark is embedded in the digital image. The reversible digital watermark includes an identifier or other payload information. The embedded digital image is then distributed to a centralized identification document production facility. The digital watermark is removed from the digital image and the identifier is obtained. The identifier is used to access information associated with the human subject. The associated information can be embedded in the digital image prior to printing the digital image on an identification document. The digital watermark can also be used to authenticate the digital image, e.g., to verify an expected distribution source or image capture location.
This disclosure also provides methods and systems for reading differently hidden information from identification documents. In one implementation an identification document includes first information steganographically embedded in a photograph or background. A preferred form of steganography is digital watermarking. The first information is typically printed on identification document (e.g., on a substrate or laminate layer). The document further includes second information hidden on a document layer. For example, the second information is conveyed through surface topology of a laminate layer. An optical sensor reads the first information, while non-visible light scanner, e.g., a laser, reads the second information.
The foregoing and other features and advantages of the present invention will be even more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.